In order to provide high quality electric power service, it is desired to monitor the quality of the electric signals at different locations along an electrical power network. Particularly it is desired to know the waveform of the current and voltage at various network locations at the time at which a specific event occurred (e.g., a blackout) in order to understand the cause of the event.
U.S. Pat. No. 5,736,847 to Van Doom et al., the disclosure of which is incorporated herein by reference, describes a power meter for reading and displaying electrical power waveforms and calculating parameters of the waveforms. These parameters may include, for example, current, voltage, power, phase relationship between waveforms of a multi-phase signal, power factor, frequency, kilo watt hour (KWH), kilo Volt Amper reactive energy (KVAR), KVARH, KVA and KVAH.
In some cases, however, on-line display of the waveforms and/or deduced parameters is not sufficient. U.S. Pat. No. 6,675,071 to Griffin, Jr. et al., the disclosure of which is incorporated herein by reference, states that it would be desired to implement a power quality device that captures all the voltage waveform data at all times, but that such device is impracticable due to the large amounts of data involved. At a sampling rate of 32 samples per cycle, 1920 samples are collected per second. For three-phase meters that measure both voltage and current waveforms, the data is 6-8 times as much. The U.S. Pat. No. 6,675,071 patent suggests detecting variations in the waveforms and capturing data relating to the times of the detected variations.
U.S. Pat. No. 6,671,654 to Forth et al., the disclosure of which is incorporated herein by reference, describes an electronic device for measuring power parameters from a power distribution system. As in the '071 patent, data is stored only when an event occurs (e.g., an abrupt change in the current or voltage).
U.S. Pat. No. 6,639,518 to Curtis, the disclosure of which is incorporated herein by reference, describes a circuit monitor which monitors one or more metered values in an electrical circuit. The monitor includes a system for selecting alarm set-points.
Some monitoring systems store only the root mean square (RMS) and/or power of the electrical signals. This data, however, is not always sufficient to determine the exact nature of problems.